1. Field of the Invention
The present invention relates to a milling tool used as a rotating bit in a machine tool such as a machining center.
2. Description of the Related Art
There has been known end mills in which a cutting insert having a shape of an approximate parallelogram is fitted into a tip part of a holder used as a rotating tool in machining with machine tools such as machining centers. The cutting insert is fitted into the holder so that a long side is disposed at the periphery of the end mill as a main cutting edge, a short side is disposed at the tip of the end mill as a flat cutting edge, and the main cutting edge slants at an angle, called the axial rake angle, to the axis of rotation of the holder.
Conventionally, a cutting insert in a plan view of which a main cutting edge is formed in a straight line have been used. However, when the cutting insert is fitted in a holder with an axial rake angle as described above and shoulder work is carried out, the machined wall face is not flat and becomes a convexly curved face swelling outward with progress from the sides toward the center so that the part machined with the central vicinity of the main cutting edge projects slightly outward. In machining where flatness is important, the machined wall face becoming a convexly curved face projecting with progress from the sides to the center like this not only impairs the quality of the appearance of the product, but also, because a re-finishing step becomes necessary, entails excessive machining time and machining cost, and has been problematic from the point of view of machining efficiency.
In this connection, to resolve this problem and obtain a smooth machined surface, in Japanese Examined Patent Publication JP-B2 3085686 it is disclosed that in a plan view of a cutting insert itself a main cutting edge swells outward, thereby it is made possible to form into a straight line a sectional form of a locus of rotation of the main cutting edge of the end mill which is cut by an imaginary plane which includes an axis of rotation and is parallel therewith and therefore a machined wall face after machining can be formed to be not so uneven.
However, when a milling tool fitted with a cutting insert whose main cutting edge swells as disclosed in Japanese Examined Patent Publication No. 3085686 is used, although when the dimensions of the cutting insert are accurately controlled and the cutting insert is mounted to the holder properly it is possible to control the locus of rotation of the main cutting edge to an accurate cylindrical shape right to the cutting edge ends, in practice, due to variation of the fitting accuracy between the holder and the cutting insert and variation of dimensional accuracy, a difference often arises between the turning radii of the ends of the main cutting edge of the cutting insert fitted in the holder, and there has been the problem that when multi-cut machining in shoulder work or grooving work is carried out in this state, large steps are formed at the joins between passes in the machining of multiple passes in the machined surface. And, in practice bending of the holder occurs during machining due to cutting load, and in this case, even if the cutting insert was mounted in a position such that the locus of rotation assumes the ideal cylindrical surface shape, steps will be still formed in the machined wall face. That is, obtaining a machined face with absolutely no steps has been, in practice, almost impossible.